| ( | 400 | ( | 1 | − | 3 100 | ) | + | 800 | ( | 1 | − | 1 20 | ) | ) | y | − | 600 | − | 5600 | = | 3558 |
| 方程左边 = | ( | 400 | ( | 1 | − | 3 100 | ) | + | 800 | ( | 1 | − | 1 20 | ) | ) | y | − | 6200 |
| ( | 400 | ( | 1 | − | 3 100 | ) | + | 800 | ( | 1 | − | 1 20 | ) | ) | y | − | 6200 | = | 3558 |
| 方程左边 = | 400 | ( | 1 | − | 3 100 | ) | y | + | 800 | ( | 1 | − | 1 20 | ) | y | − | 6200 |
| = | 400 | × | 1 | y | − | 400 | × | 3 100 | y | + | 800 | ( | 1 | − | 1 20 | ) | y | − | 6200 |
| = | 400 | y | − | 12 | y | + | 800 | ( | 1 | − | 1 20 | ) | y | − | 6200 |
| = | 388 | y | + | 800 | ( | 1 | − | 1 20 | ) | y | − | 6200 |
| = | 388 | y | + | 800 | × | 1 | y | − | 800 | × | 1 20 | y | − | 6200 |
| = | 388 | y | + | 800 | y | − | 40 | y | − | 6200 |
| = | 1148 | y | − | 6200 |
| 1148 | y | − | 6200 | = | 3558 |
| 1148 | y | = | 3558 | + | 6200 |
| 1148 | y | = | 9758 |
| y | = | 9758 | ÷ | 1148 |
| = | 9758 | × | 1 1148 |
| = | 17 | × | 1 2 |
| y | = | 17 2 |
| y | = | 8.5 |